The present invention relates to a contact welding detecting device for a relay, which detects welding of each contact of a relay having a first contact and a second contact, the first contact serving to permit/break the transmission of a negative DC input potential to an output side and the second contact serving to permit/break the transmission of a positive DC input potential to an output side.
In EN standard in Europe, a specific design standard that a safety relay has to satisfy is prescribed. In the case one of contacts of a relay is welded, a safety relay guarantees the other contact to be opened by a mechanical safety system. Nowadays, the safety relay is widely used as an output module that outputs electric power of a safety system constructed by, for example, using a safe PLC. Known systems constructed by using the safety relay disclosed in JP-A 2005-025479: JP-A 2005-025260; and JP-A 2005-004557.
FIG. 3 is a circuit diagram showing a related output module that outputs the electric power by using the safety relay. The safety relay sR includes contacts ‘a’ and ‘b’ and a coil. L. The contacts ‘a’ and ‘b’ are controlled to be simultaneously opened or closed by a current which flows through the coil L. However, for example, when the contact ‘a’ is welded, the contact ‘b’ is kept to be opened on the basis of a safety system ‘x’ included in the safety relay. Accordingly, even though microprocessors MPU-A and MPU-B output ON commands at the same time to make a sufficient current flow through the coil L, the current does not flow through a light-emitting diode D1 included in a photo-coupler C1. As a result, an output signal S indicating an examination result on the safety relay becomes in a High state indicative of an abnormal state.
According to the above-described related configuration, it is possible to use only one photo-coupler (photo-coupler C1 in FIG. 3) for directly detecting whether or not the relay is opened or closed and it is also possible to examine whether or not each of contacts is welded when the relay is closed (ON state) by the current flowing through the coil L.
However, since a safety relay should include a mechanical safety system as described above, it is difficult to maintain a small size and low price for the output module in the case of constructing an output module using a well-known safety relay as compared with the case of constructing the output module using a general relay.